The location of WLAN devices can be performed by various means. Of particular interest for this disclosure is the location of devices that are based upon the IEEE 802.11 technology, commonly known as Wi-Fi. Various methods can be used to locate an access point (AP), or a station (STA). These methods may be classified as active, passive and combined active and passive. In an active location scheme, a device that is determining the location or range, the measuring device, transmits certain packets to the device being located, the target device, and the common method is to measure the time of arrival (TOA) of the response from the target device and compare that to the time of departure (TOD) that the packet was transmitted by the measuring device. In a passive location scheme, a measuring device simply monitors the TOAs of non-stimulated transmissions from the target device.
In such location systems it is common to use multiple measuring devices to determine the location. In such a scheme simultaneous TOA and/or TOD measurements are taken by different measuring devices situated at different points and the location of the target device calculated. For example, in a passive location scheme, the TOA of a transmission from the target can be simultaneously received at several sites. The difference in the TOAs between two sites is known as the time difference of arrival (TDOA). The TDOA is related to the difference in path lengths between the target and each receiving site, and for each pair of receivers the TDOA results in a hyperbola along which the location of the target lies. The addition of a third site will provide a second hyperbola and the location of the target will be indicated by the interception of the two hyperbolas.
In an active location scheme, TOD may be measured for a packet that is transmitted from the measuring station addressed to the target station. The TOA of the response from the target station at the measuring station is then also measured. If the turnaround time for the target station to receive the packet from the measuring station and to start to transmit the response is known, then the time difference at the measuring station between the TOA and the TOD, minus the turnaround time at the target station will be directly proportional to twice the distance of the target station from the measuring station. For example, if the target station is a wireless device based upon IEEE 802.11 technology, and if the packet transmitted from the measuring station to the target station is a data packet, the response from the target station will normally be an acknowledgement (ACK) packet. If the packet transmitted from the measuring station to the target station is a control packet, for example a ready-to-send (RTS) packet, then the response from the target station will normally be a clear-to-send (CTS) packet. In these two examples, the turnaround time at the target station is defined in the IEEE 802.11 standard as the short interframe spacing (SIFS) which is a preset value. Hence, the time delay, td, between the measuring station and the target station may be determined from the calculation td=(TOA−TOD−SIFS)/2 and the distance between the measuring station and the target station is then td/c, where c is the speed of light. This method of estimating the distance to a target station by measuring the TOD and TOA and accounting for the turnaround time is known in the art.